Late Period on Metformin??

I was diagnosed with PCOS 2 months ago. I have regular 28 day cycles. Normal bleeding. Little to no PCOS symptoms besides hormonal imbalances I guess. I assume I have insulin resistance so my doctor prescribed Metformin 500mg twice a day. My first period since taking the medicine lasted about 10 days and was kind of heavy. (I normally bleed 3-5 days with normal bleeding). This is supposed to be my second period since starting the meds and my period is 3 days late. I see that it’s a common issue but among the women with irregular periods. I have clockwork 28 day cycles and never had a late period. I took a first response pregnancy test after my period was a day late and got a BFN. Is this normal? Should I wait to test again? Or just wait on my period?